The invention relates to adjustable steering column assemblies of the kind which include a first shroud portion, a second shroud portion and a clamp mechanism that includes a bracket secured to a fixed part of a vehicle and in which the first and second portions are releasably engaged by the clamp mechanism to selectively permit an amount of relative movement between the first and second portions that is required during adjustment of the steering column assembly.
Steering column assemblies are known which include a clamp mechanism that relies solely on friction when locked to prevent unwanted movement of the first and second parts of a steering column assembly. The clamp mechanism can be unlocked and locked by movement of a locking lever or, sometimes, electrically using a motor or perhaps hydraulically or pneumatically.
By releasing the clamp mechanism and telescopically sliding the first portion over the second portion the length of the shroud can be altered, altering the reach position of an associated steering wheel. By moving both portions together up and down relative to the clamp mechanism the rake can be adjusted. In some assemblies only one of reach and rake may be adjusted.
The clamp mechanism may be attached to a bracket (known as a Rake Bracket) which is connected to the vehicle through one or more frangible fasteners. In the event of a crash causing a high load to be applied to the wheel the fasteners can sever allowing the bracket and the clamp mechanism to move relative to the vehicle, in turn allowing whichever part of the shroud it is secured to also to move so the wheel can collapse away from the driver.
To control the rate of collapse it is known to provide an energy absorbing device such as one or more straps which engages the vehicle body at one end and the bracket at the other. As the bracket moves relative to the vehicle it causes the strap to deform beyond its elastic limit and this deformation absorbs some of the energy, controlling the rate of collapse.
In a common arrangement, shown in FIG. 1 of the accompanying drawings, the energy absorbing device comprises two members which each comprise a strap 60, 61 anchored to two metal blocks (known as Capsules) which form the frangible connectors which enable the bracket to break away in crash and move forward in the vehicle under the impact of the driver's torso on the steering wheel and column upper structure. Note that the force of the impact is transferred from the column upper structure to the Rake Bracket by the Clamp Mechanism. The Capsules, which are usually positioned symmetrically on either side of the column centre-line, commonly each have a single vertical hole passing approximately through their centre through which a fixing bolt secures it to the vehicle structure.
Generally, the energy 60, 61 straps are threaded through additional blocks having labyrinthine apertures 70, 71 (sometimes called Anvils). The Anvils are fixed in the Rake Bracket and are situated ahead of the Capsules. When the Rake Bracket breaks away from the capsules, the Energy Straps are progressively dragged through the Anvils and deformed by them in the process. This causes a proportion of the impact energy to be absorbed more or less evenly over the collapse stroke of the column. The result is to reduce the peak force acting on the driver's torso. This arrangement is known to be efficient, cost effective and easy to tune by selecting the width, thickness and material properties of the Energy Straps.
However there are instances, especially if a column-mounted electrical power assistance system (known as an EPS) is fitted to the lower end of the column, where the area needed to accommodate the above described Energy Strap configuration in the top plate of the Rake Bracket means that the latter will clash with some parts of the EPS before the intended crash stroke can be completed